The present invention relates to an electrophotography apparatus and an electrophotographic process and, more particularly, to an improved electrophotography apparatus and electrophotographic process for developing a positive image from a positive or negative film.
Generally, a microfilm is used to record and preserve various images, an electrophotographic process being utilized to record microfilm images on paper.
In an electrophotography unit, light passing through a film is applied onto a photoconductive drum charged by a main charger and rotating in a predetermined direction, thereby to form a static latent image on the drum. This static latent image is developed into a visible image by means of a developing system, and then is transferred onto paper or the like by means of a transfer charger. Thereafter, the paper onto which the image has been transferred is separated from the photoconductive drum by a separation charger.
Residual toner on the surface of the photoconductive drum from which the paper has been separated is removed by a cleaning unit, and any residual charge thereon is discharged by a discharge lamp. Thereafter, the drum is once again charged by the main charger, and the above process is repeated.
Such an electrophotography developing method includes a normal developing method and a reversal developing method. In the case of the former method, toner having a polarity opposite to that of the charge is attracted to a charged portion of the static latent image on the drum, for development to take place. In the case of the latter method, toner having the same polarity as that of a high-potential portion is attracted to an exposed portion of the static latent image on the drum, for development to take place.
In order to ensure that the photoconductive drum can be used repeatedly, it is essential that any residual transfer toner be removed from the drum surface, upon completion of each transfer process. For example, in the case of the normal developing method, if this cleaning process is omitted and a discharging process is performed immediately after the transfer process, the result is as follows:
When the photoconductive drum is used repeatedly, the residual transfer toner becomes charged once again when the next charging process is performed. Then, when a new developing process begins, the residual potential of the drum after exposure ends up being below the developing bias. For this reason, residual toner on the exposed portion does not be removed during development, but instead remains on the drum. Then, when the subsequent transfer process takes place, the toner is transferred from the drum onto the paper, thereby giving rise to ghost images, background fogging, and other flaws, all of which degrade image quality. Therefore, in order to ensure that the photoconductive drum can be used repeatedly, it is essential that a cleaning process be carried out between the image transfer and charging processes.
However, in order to provide a cleaning unit for carrying out the cleaning process, it is necessary that installation space be provided in the electrophotography unit, with the result that the entire unit must be enlarged. In addition, since a cleaning blade now presses against and slides along the drum, the drum is therefore subjected to a degree of mechanical stress, which may cause damage thereto, or result in the toner and other substances being pressed firmly onto the surface of the drum, giving rise to the formation of a film thereon, with the risks that this poses to image quality.
In addition, the residual transfer toner removed by the cleaning unit must be collected in a collecting vessel and discharged when it reaches a predetermined volume, which entails drawbacks as concerns operation efficiency and energy saving.
For the reasons stated above, an apparatus has been manufactured wherein the cleaning unit is omitted and the photoconductive drum is rotated twice in the course of a single image forming process. That is, the developing system is alternately operated for the developing process and the cleaning process during each drum rotation.
For example, when the charging process is performed, the photoconductive drum is uniformly charged to have surface potential V.sub.0, and is exposed in the exposing process to form a static latent image. When the developing process is performed, toner having a polarity opposite to that of the charge is attracted to the charged portion of the static latent image by a developing roller biased to have a potential substantially the same as, or slightly higher than, the residual potential of the exposed discharging portion of the drum, whereby development takes place. A powder image thus developed is then transferred to a suitable medium by the transfer charger. Thereafter, the drum is electrically discharged by the discharging lamp and the discharging charger. From the beginning to the end of the entire process, the drum rotates substantially once.
Thus, bias V.sub.B of the developing roller is set such that 0 &lt;V.sub.B &lt;V.sub.0. In this case, the developing roller also serves as a cleaning unit for removing the residual transfer powder on the drum. In this manner, one recording image is formed while the photoconductive drum rotates twice.
However, such with this type of an apparatus, the circumferential length of the photoconductive drum must be greater than at least the length of the recording image in a process wherein the drum is used repeatedly. In other words, if the circumferential length of the drum is less than the length of the recording image, the trailing end of the image on the drum is still in the developing process when the leading end of the image thereon reaches the position of the developing roller. As a result, the developing roller cannot serve as the cleaning means, and hence the residual transfer powder on the leading end of the image on the photoconductive drum will not be removed therefrom.
For this reason, this apparatus has a drawback in that the circumferential length of the photoconductive drum, and, by inherence, the size thereof, must be increased. In addition, since one of each two rotations is for enabling the cleaning process to be carried out, the usage efficiency of the photoconductive drum is therefore only 50%.
In addition, since the rotation al speed of the drum must be reduced because of the above reasons, it is necessary to provide two bias power sources for applying different biases to the respective developing rollers.
Furthermore, the electrophotography unit can perform the normal and reversal developing methods, both described above incorporates two developing units, these being operated selectively in accordance with the development method (normal development and reversal development) chosen.
A roll is provided within the developing roller of either developing unit. The roll must be rotated by a prescribed angle in order to operate the developing unit. Hence, two driving systems are required for the developing units, respectively, for rotating the rolls. Further, a driving mechanism is required for driving the first developing unit or the second developing unit, thus selecting the normal or reverse developing method. If only one developing unit sufficed, one driving system and the driving mechanism should be unnecessary. In view of this, the conventional electrophorography apparatus is somewhat complicated.